1. Field of the Invention
This invention relates to an apparatus for the synthesis of ammonia. More particularly, this invention provides an apparatus for an ammonia synthesis process in which the energy released in the process is recovered at high temperature levels in a system which is more compact and less costly than with previously known types of apparatus.
2. Description of the Prior Art
Ammonia is produced commercially today by continuous processes which involve the seemingly straightforward reaction between stoichiometric amounts of nitrogen and hydrogen: EQU N.sub.2 +3H.sub.2 .fwdarw.2NH.sub.3
In practicing such processes, a gaseous mixture containing nitrogen and hydrogen is passed sequentially over one or more catalyst beds containing, for example, granular iron or promoted iron catalyst, at elevated pressure and temperature.
Commercial processes are known for carrying out the synthesis over a wide range of pressures, from about 20 to 1000 atmospheres, but most modern commercial processes employ pressures in range of about 60 to 300 atm.
The reaction is exothermic--heat is released as the reaction proceeds, therefore the equilibrium conversion of hydrogen and nitrogen to ammonia is greater at lower temperatures. However, at any given gas composition, the reaction rate velocity constant decreases as the temperature is lowered, so that as a practical matter, the temperature must be maintained at a high enough level to permit the synthesis of acceptable quantities of ammonia product in a reasonably short time. This is true even with the acceleration of the reaction rate achieved with a catalyst.
For minimum catalyst volume, the temperature at each point in catalyst would be controlled at the level at which the reactivity and the equilibrium driving force corresponding to the composition at that point are balanced to achieve the maximum rate of ammonia formation. In such an ideal system, both the temperature and the rate of heat removal would be highest at the inlet of the catalyst, both gradually decreasing to lower levels at the outlet.
Older commercial processes attempted to approach these conditions for minimum catalyst volume as much as practical, with varying degrees of success, by imbedding indirect heat transfer surfaces throughout the catalyst bed by which heat could be transferred by indirect heat exchange to a cooling fluid such as incoming feed gas or other cooling media.
It was eventually discovered, however, especially for larger plants, that as a practical matter the costs of fabrication, maintenance, catalyst loading, and catalyst unloading of such systems was unnecessarily costly, and that a more practical and more economical approach is to employ a series of two or more adiabatic beds with successively lower outlet temperatures. Most modern processes employ the latter approach.
In such processes, as the gaseous mixture passes through each bed, the ammonia concentration increases as hydrogen and nitrogen react, and the temperature of the gas is also increased by the exothermic heat of reaction, until the ammonia concentration and temperature approach equilibrium conditions.
To achieve further conversion, the gaseous mixture is withdrawn from the first bed, cooled to a lower temperature at which the equilibrium concentration of ammonia is greater, and then introduced to the second bed, where the phenomena occurring in the first bed are repeated, except at higher ammonia concentration levels and lower outlet temperatures. In some processes, additional beds are employed in the same manner to obtain still greater ammonia concentrations.
Two general methods are used to cool the gas leaving a bed before sending it to another bed. One method is to quench directly the gas leaving a bed by mixing with it a part of the feed gas which, because of its lower temperature, results in a mixture of a lower temperature than that of the effluent before mixing. The other is to cool the gas by indirect heat exchange with another fluid.
In some processes all the catalyst beds and all the devices for cooling the gases leaving the beds are contained in a single pressure vessel. In other processes, each bed and each cooling device is contained in a separate pressure vessel. And in still other processes at least one of two or more pressure vessels may contain a combination of two or more of these components.
In many designs, the catalyst and sometimes also one or more exchangers are held in a cartridge, basket, or other type of container which is disposed inside a pressure shell, the pressure shell being shielded from the hotter catalyst bed and the high temperature of the effluent gas passing the colder feed gas through an annular space between the internal container and the pressure shell. In such designs the inlet and outlet connections are often located at the same end of the pressure shell, usually at the bottom end. Examples of such designs are described in U.S. Pat. Nos. 3,851,046 and 3,721,532 of Wright, et al., the disclosures of which are specifically incorporated herein by reference.
U.S. Pat. No. 4,554,135 of Grotz, et al., the disclosures of which are specifically incorporated herein by reference, describes a means for close coupling the inlet and outlet connections at the bottom of a reactor to a horizontally-disposed heat exchanger used to transfer heat from the hotter reactor effluent gas to the colder reactor feed gas in a manner which avoids the exposure of the reactor pressure shell, the exchanger pressure shell, or the pressurized connecting conduit to the high temperatures of the catalyst bed and the reactor effluent gas.
A number of processes and apparatus arrangements are known in which an adiabatic reactor is preceded by a heat exchanger in which the effluent of a preceding reactor is cooled by indirect heat exchang with high temperature boiling water to generate high pressure steam and followed by a similar high pressure steam generator for cooling the effluent of that reactor. Several examples are described in U.S. Pat. Nos. 4,510,123, 4,624,842, 4,744,966, and 4,867,959 of Grotz, the disclosures of which are specifically incorporated herein by reference.
In FIG. 2 of Grotz '959, for example, the third reactor, catalytic converter 238, is preceded by high pressure steam generator 234, recovers energy from the effluent of the preceding reactor, catalytic converter 228, and is followed by high pressure steam generator 246, which recovers energy from the effluent of catalytic converter 238. The two streams of high pressure steam generated in steam generators 234 and 238 are combined into a single stream in conduit 230.
In carrying out a process having the steps described above with conventional known apparatus, each high pressure steam generator is a separate heat exchanger, each with its own pressure shell, feedwater supply system, level control system, safety relief system, blowdown system, and other instrumentation, each entailing duplicate costs. A need exists therefore for a practical and economical means of eliminating this duplication of costs and complexity for performing the corresponding duplicate in comparison to those required with known apparatus.